The interesting little Rhodope veranyii, which has no odontophore, has been associated by systematists both with these simplified Opisthobranchs and with Rhabdocoel Planarians.
For a comparison of various Opisthobranchs, Aplysia will be found to present a convenient starting-point.
It is one of the more typical Opisthobranchs, that is to say, it belongs to the section Tectibranchia, but other members of the suborder, namely, Bulla and Actaeon (figs.
The great development of the parapodia seen in Aplysia is usual in Tectibranchiate Opisthobranchs.
The typical character is retained by the heart, pericardium, and the communicating nephridium or renal organ in all Opisthobranchs.
Opisthobranchs provided in the adult state with a shell and a mantle, except Runcina, Pleuro.
External to the branchia are seen ten club-like processes of the dorsal wall, these are the " cerata " which are characteristically developed in another sub-order of Opisthobranchs.
In the marine Streptoneura they are ectodermic projections which ultimately fall off; in the Opisthobranchs they are closed pouches; in Paludina and Bithynia they are canals as in Pulmonata.
Neomenia, mentioned first by Michael Sars in 1868 under the name Solenopus, was afterwards included among the Opisthobranchs by J.
The only other molluscs in which they are exactly represented are the curious Opisthobranchs Phyllidia and Pleurophyllidia (fig.
In many of the Nudibranchiate Opisthobranchs the nervous system presents a concentration of the ganglia (fig.
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